1. Field of the Invention
This invention relates generally to windings for electrical machinery, and more specifically, to a superconductive field winding for the rotor of a dynamoelectric machine.
2. Description of the Prior Art
The idea of using superconducting windings in high power electrical machinery is well known in the art. Only recently, however, have materials been found which are capable of remaining in the zero resistance condition at high transport current densities and large direct current magnetic fields. For example, the development of the intrinsically stable multifilamentary superconductor has made it possible to build stable superconducting field windings with relatively high direct current densities. The use of superconductive direct current field windings allows a considerable increase in the field magnetomotive force generated by the windings and greatly increased flux densities in the active air gap of the machine. This increase in flux density obtains considerably increased power density and consequential reduction in the weight and volume of the machine. The size and weight reductions make superconducting machines attractive for such applications as electric drive ship propulsion systems. Also, higher ratings for turbine generators can be obtained without prohibitive increases in frame size.
It is useful to consider the phenomenon of superconductivity and the related properties of superconductive materials in order that the present invention may be clearly understood. Superconductivity is the state in which some metals offer no resistance to current and therefore do not generate heat as do normal conductors. The resistance at superconducting temperature is not merely extremely low, it is exactly zero. Superconductivity occurs only at very low temperatures; the temperature is different for each material and is known as the transition or critical temperature, T.sub.c. At the transition temperature, which is a few degrees above absolute zero, there occurs a thermodynamic transition into the superconducting state. The transition temperature, in the absence of a magnetic field, is 3.7.degree. Kelvin for tin, 7.3.degree. Kelvin for lead, and 8.degree. Kelvin for niobium. For further information on specific properties, see National Bureau of Standards Technical Note 724, "Properties of Selected Superconductive Materials," 1974 Supplement, published by the U.S. Dept. of Commerce.
In addition to temperature, the strength and geometry of magnetic fields affects superconducting materials. A material will suddenly lose its superconductivity in a high strength magnetic field, even a self-generated field, when it reaches a value known as its critical magnetic field, H.sub.c. There also exists a critical electrical current density, J.sub.c, which is dependent upon both the temperature and the magnetic field. The three parameters T, H, and J define a three dimensional surface which separates the superconducting and normal regions as illustrated in FIG. 1 of the drawing. For a given temperature (shaded region of FIG. 1) a superconducting coil will have some design load line as illustrated and an operating point P' chosen to be less than the critical point P, where a normal transition occurs. This return to the normal state is referred to as a quench. It should be understood that while the shape of the critical curves for any superconductive material is generally as indicated in FIG. 1, the intercepts at the axes are determined by the properties of the material selected.
Superconductors which are suitable for high current density, high field applications (usually called type II or hard superconductors) are subject to instabilities, where a small disturbance in operating conditions can cause a quench, even though the critical current density, magnetic field, or temperature is not exceeded except in a very small region of the coil. The current carrying capability of a single superconductor is limited by the maximum field seen at any point on the conductor. The current rating of a superconductive winding will therefore be greatly reduced by a high flux concentration even in a small region of the winding. Thus a serious problem involved in superconducting windings is the maintenance of superconductivity under magnetic field conditions which tend to destroy superconductivity. An equally important consideration is that of obtaining the maximum useful external field available from a given amount of superconductive material, once operating stability is achieved.
A known technique for preventing premature normalization due to non-uniform field conditions is to divide the superconductor into many fine filaments embedded in a high electrically and thermally conductive material such as high purity copper. The entire conductor is usually twisted about its axis to reduce eddy current losses. The copper dissipates heat from any small portion of the superconductor that may happen to normalize, thus preventing a stray normalization from heating the strands and causing destruction of the superconductivity throughout the coil. Such a superconductor has been described by G. H. Morgan in "Theoretical Behavior of a Twisted Multicore Superconducting Wire in a Time-Varying Uniform Magnetic Field," Journal of Applied Physics, August 1970, Vol. 41, page 3673. The amount of copper used in this technique is usually between one and three times the amount of superconductor. Although the use of copper increases operating stability, it has the undesirable effect of significantly reducing the overall current density, particularly when the ratio of copper to superconductor is increased to a proportion greater than 3:1. Thus there exists practical limitations on the use of the copper dissipation technique.
It is known in the art to reduce the current flow in superconductors disposed within regions of high magnetic field intensity by means of resistance elements so that critical current density values are not exceeded. However, a winding structure for the rotor of a dynamoelectric machine which utilizes resistance elements in this manner has not been disclosed.
The known way to connect a superconducting field winding is to connect all parts of the winding in series with all conductors carrying the same current density of a predetermined magnitude. The magnetic field produced by the series connected winding arrangement is not uniform, and the current in the winding may be increased only until current flow in the point, or points, at which the field is greatest reaches the critical value for the destruction of superconductivity. It is the principal object of the present invention, therefore, to provide a superconductive direct current field winding structure for the rotor of a dynamoelectric machine such that at points of high field intensity, the current density in the superconductive portions may be reduced, while at points of lower field strength it may be increased, thereby preventing normalization while producing a net increase in the useful external field.